1. Field of the Invention
This invention relates to a distance measuring apparatus for measuring a distance from its position to a subject of interest and in particular to such a distance measuring apparatus as a range finder for use in a camera and the like.
2. Background of the Invention
As a distance measuring apparatus in an autofocus system of a compact-sized camera and the like, use has been mainly made of a passive type dual image coincidence system utilizing external light. However, in such a passive type dual image coincidence system, in which the distance from its position to a subject of interest is determined when the coincidence in position of two images is obtained, it is required to use a movable mirror for varying the position of one image with respect to the other image. The use of such a movable mirror has been a cause of poor durability of prior art distance measuring apparatus. The dual image coincidence system is strongly dependent upon the condition of a subject of interest since it carries out the measurement of distance on the basis of the contrast information of a subject of interest such as a subject to be photographed. Thus, prior art apparatus have been disadvantageous in low capability of measuring distance for the case in which a subject of interest has a low contrast or for the case in which a subject of interest is in a dark place. Moreover, such a prior art system having a movable part has tended to be complicated in structure, requiring time-consuming adjustments.
Use has also been made of an active type triangulation distance measuring system, according to which, since the light to be used for measurement is emitted from the apparatus itself, the problem of dependency on the conditions of a subject of interest is obviated. However, even in such an active system, if there is provided a movable part such as a pivotal light-emitting or light-receiving section, the above-described disadvantages of poor durability and complicated adjustments cannot be solved.
In view of the above, there has been proposed an improved active type triangulation distance measuring system having no movable parts, as shown in FIG. 1. As shown, there is provided a light-emitting section 1 which emits light such as infrared light, which is then reflected by a subject of interest 2, e.g., 2a, 2b, 2c and 2d. The reflected light then impinges upon a light-receiving section 3 including a plurality of photocells, four cells 3a, 3b, 3c and 3d in the embodiment shown. Then, the distance to the subject 2 may be obtained by determining which of the light-receiving elements has received the reflected light.
In the distance measuring system shown in FIG. 1, the disadvantages of poor durability and complicated adjustments are obviated. However, it suffers from a disadvantage of limited resolution in distance measurement because of the quantized structure of the light-receiving section 3. For example, in the case where the light receiving section 3 is comprised of four light-receiving elements 3a-3d, as shown in FIG. 1, even if the boundary between two adjacent elements is included, the maximum number of levels results in seven and this number may be further reduced when error is taken into account.
Another form of the active type distance measuring system has been the one using ultrasonic waves. In accordance with this ultrasonic system, an ultrasonic wave is radiated toward a subject of interest and the reflected wave from the subject is received by the system, thereby the distance between the system and the subject is determined by the time expended for going and returning trip. In this system, measurement is carried out by a pure electrical processing, which is rather simple; however, a relatively large power supply is required to obtain a high power ultrasonic radiation. Thus, a power supply incorporated in a compact-sized camera would be insufficient in generating an effective ultrasonic radiation. Moreover, in order to prevent the lowering of accuracy in distance measurement from occurring due to reflection from other subjects than the one of interest, directionality of the radiation must be enhanced, which, in turn, requires a larger area for radiating or receiving a ultrasonic wave. This also presents a problem for application to compact-sized cameras.